A liquid crystal display device includes a thin film transistor substrate (TFT substrate) for displaying an image on a display surface by controlling transmission/non-transmission of light from a backlight unit through a liquid crystal material provided between the backlight unit and the display surface. The thin film transistor substrate includes a transparent substrate, a gate electrode arranged on the transparent substrate, a gate insulating film arranged on the gate electrode, a semiconductor layer arranged on the gate insulating film, a source electrode and a drain electrode arranged on the semiconductor layer, a protective insulating film arranged on the source electrode and the drain electrode, and a pixel electrode connected to the source electrode or the drain electrode. A part of the semiconductor layer, which is formed between the source electrode and the drain electrode, corresponds to a channel forming region in which a channel is to be formed.
FIG. 14 is a conceptual diagram of a pixel circuit 1401 formed of a thin film transistor 1402. The pixel circuit 1401 provided in the thin film transistor substrate controls transmission/non-transmission of light from the backlight unit provided so as to be opposed to the thin film transistor substrate.
One of a source and a drain of the thin film transistor 1402 is connected to a video signal line 1403, and the other of the source and the drain is connected to a pixel electrode 1404. In an in-plane switching (IPS) mode liquid crystal display device, the pixel electrode 1404 and a common electrode 1405 are formed in the thin film transistor substrate.
A video signal and a reference potential are applied to the video signal line 1403 and the common electrode 1405, respectively. Further, ON/OFF of the thin film transistor 1402 is controlled by a gate signal, to thereby change a potential difference between the pixel electrode 1404 and the common electrode 1405. A liquid crystal material 1406 sealed between the thin film transistor substrate and a color filter changes its orientation in accordance with the change of the potential difference (generated in a direction parallel to the surface of the thin film transistor substrate), and thus the transmission/non-transmission of light from the backlight unit is controlled.
Japanese Patent Application Laid-open No. 2009-122299 discloses a liquid crystal display device in which a counter voltage signal line for supplying a reference signal to a counter electrode is formed along a running direction of a gate signal line so as to overlap with the gate signal line, to thereby improve the aperture ratio.